1. Field of the Art
The present invention relates to injection molding apparatus controllers, and more particularly, to a controller for controlling the transition between the injection stage and the packing stage of an injection molding apparatus.
2. Discussion of the Related Art
As is well known, injection molding apparatus operate to form a solid, plastic object by injecting molten plastic into a mold cavity, the mold cavity defining the shape of the solid object. The process of creating an injection molded object is broadly defined in four process stages. The first stage is the plastication stage, in which granulated plastic material is introduced and melted within a cylinder. The second stage is the injection stage, in which the molten plastic material is forced from the cylinder into an injection mold that is in fluid communication within the cylinder. Next is the packing stage which begins once the injection mold has filled with molten plastic. During this stage, additional molten plastic is forced into the mold to accommodate shrinkage as the plastic cools in the mold cavity. Finally, the last stage is the holding stage. In this stage, the plastic is maintained at a substantially constant pressure as the plastic solidifies to control plastic properties, such as density and resilience.
To more particularly describe the injection and packing stages of this process, during the injection stage a ram is disposed within the cylinder and is controlled to move within the cylinder to force the molten plastic out of the cylinder and into a mold cavity. The velocity of the ram is closely controlled to control the flow rate of the molten plastic into the mold cavity. Because plastic is flowed to sequentially fill spaces within the mold cavity, the velocity of the ram is preferably controlled in accordance with the interior cavity space being filled. Filling small areas too quickly can result in burning of the plastic, while filling the spaces too slowly may result in voids. It is appreciated that either of these outcomes may result in products that are undesirable and possibly unusable.
When, however, the mold fills and the apparatus enters the packing stage, the velocity of the ram falls to substantially zero. At this time, the process control is switched to pressure control, whereby a substantially constant pressure (of a predetermined value) is maintained on the injection ram. Just as it is important to closely control the ram velocity during the injection stage, it is equally important to control the ram pressure (and therefore the pressure applied to the plastic) during the packing stage. Pressurization during this stage affects the composition of the solidified plastic object, and over or under pressurizing the plastic may result in an undesireable or unusable part.
Numerous types of position sensors are known for detecting the position of the ram during the injection stage. By evaluating the change in ram position over time, the ram velocity may be ascertained. Similarly, pressure sensors are typically located in either or both the mold cavity and the injection cylinder (housing the ram), and these pressure sensors may be utilized to monitor the plastic pressurization during the packing stage.
In the prior art of which applicants are aware, the transition from the injection stage to the packing stage is determined either by monitoring the ram position or the pressure sensors. When the ram position crosses a threshold, it is assumed that the mold is full and pressure control for the packing stage should commence. Likewise, if the pressure detected by the pressure sensors exceeds a given threshold, it is assumed that packing stage control should begin.
The problem with both of these approaches is that an undesireable pressure spike usually occurs at the injection-to-packing stage transition. That is, by the time that the appropriate conditions are sensed and packing stage control is assumed, the plastic has become unduly pressurized. As previously mentioned, such overpressurization can result in undesireable and unusable parts.
Extreme pressure spikes may be somewhat mitigated by altering the predetermined sensing or trigger points. Alternatively, when monitoring pressure sensors, lowering the pressure threshold helps moderate the problem. Often, however, if the pressure overshoot is curtailed it is found that the overall system performance or efficiency has been compromised. It is, therefore, desireable to improve the controller performance in an injection molding apparatus, as the injection molding process transitions between the injection and packing stages.